In recent years, batteries for use in portable terminals including notebook computers and mobile phones, hybrid cars, electric vehicles, and power-assisted bicycles have been spreading rapidly, and further size reduction and weight reduction have been sought. Lithium-ion secondary batteries having the advantages of a high energy density and being lightweight have been used as the batteries (rechargeable batteries) for use in these various products.
A lithium-ion secondary battery generally includes a positive electrode including a positive electrode active material allowing doping and dedoping of lithium ions, a negative electrode including a negative electrode active material allowing doping and dedoping of lithium ions, and an electrolyte. The electrodes for lithium-ion secondary batteries are usually fabricated by applying a slurry for forming an electrode on an electrode collector and drying the slurry.
Examples of the slurry for forming an electrode include the one obtained by mixing and kneading an electrode active material such as a positive electrode active material or a negative electrode active material, a binder, and a dispersion medium. Organic solvent-based binders have mainly been used as a binder and dispersion medium. For example, typically, polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE) is used as a binder and a polar solvent such as N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium.
Cellulose has a number of hydroxy groups that can form hydrogen bonds in molecules and therefore can serve as an electrode binder excellent in binding with an electrode active material and an electrode collector. That is, if cellulose is used as an electrode binder, even on the occurrence of stress on the electrode during an electrode production process, absorption and desorption of lithium ions in charge/discharge cycles, and volumetric changes of the electrode itself due to temperature changes, it is possible to alleviate the stress, to prevent separation and dropping-off of the electrode collector from the electrode active material, and to suppress reduction in adhesiveness (see Patent Document 1).
The organic solvent-based binders conventionally studied are excellent in binding between the collector and the electrode active material but, in terms of costs and in view of environmental conservation, has an issue that the use of an organic solvent is necessary when the slurry for forming an electrode is kneaded and applied. New binder compositions using no organic solvent, therefore, have been studied, and the use of a resin dissolving or dispersing in water as a binder has been attempted. When such an aqueous slurry is used as a binder, the cellulose-based compounds may be used in combination in order to improve dispersiveness of the electrode active material etc. in the slurry, to improve charge/discharge cycle characteristics, and to improve the battery capacity retention ratio (see Patent Document 2).